Fix client keepalive cycling

[johnf]

keepalive wasn't in the check_args list so it would cycle on and off on
each puppet run

[bodgit]

I'm fairly sure this change breaks the ability for the client to define any custom keepalive settings. Prior to this change you could do this:

class { 'sensu':
  …
  client_custom => {
    keepalive => {
      ...
    },
  },
  …
}

But by adding "keepalive" to check_args() and no other way to set that in the configuration they're pruned out.

[jamtur01]

@johnf Any thoughts?

[johnf]

See my comment in #257